Metallic compositions containing bismuth



Patented June 26, 1945 BISMU Franz It. Hensel, Indianapolis, Ind.,assignor to P. R. Mallory & (30., Inc., Indianapolis, Ind., corporationof Delaware Application November 2, 1943, Serial No. 508,757

No Drawing.

1 Claim.

The present invention relates to metal compositions and articles madetherefrom and more particularly to compositions and articles ofmanufacture containing bismuth.

It is known that bismuth doe not appreciably alloy with metals such ascopper or aluminum in any substantial proportion. The degree ofsolubility of solid bismuth in either copper or aluminum is less than0.1%. All copper-bismuth alloys containing more than 0.2% of copperconsist of crystals of substantially pure copper surrounded by anenvelope of substantially pure bismuth, which explains why many alloyscontaining bismuth within or adjacent to their grain boundaries are sobrittle as to be useless for practically all industrial applications.

An object of this invention is to provide useful metallic compositionscontaining bismuth.

A further object of the invention is to provide metal compositionshaving improved properties which make them particularly advantageous foruse as bearing materials, resistor materials, electric contact alloysand rectifying discs.

Another object of the invention is to provide articles of,.the typementioned having increased efficiency of operation, an increased usefullife, at a greatly reduced cost of production thereof.

More specific objects and advantages of the invention will appear fromthe following description thereof.

Broadly, the present invention contemplates the use of bismuth as animpregnating material for a number of metals, or mixtures thereof,selected from the group having a relatively high electrical conductivityand a relatively low afllnity or alloyforming tendency with bismuth,such group including copper and silver. Bismuth has a number ofoutstanding properties. One of these properties is that it changes itselectrical resistance when subjected to a magnetic field. The resistanceof bismuth increases in a transverse magnetic field. For a given fieldstrength the percentage change increases with a decrease in temperature.At a given temperature the percentage change in resistance increaseswith an increase in field strength; for example, at a constanttemperature of 18 C., the percentage change in resistance is 12% at 4000gausses, 32% 'at' 8000 gausses and 104% at 20,000 gausses.

For a number of electrical control applications, the high resistivity ofpure bismuth (about 119 microhms-cm.) interferes with satisfactoryoperation. It therefore becomes a problem to provide a material whichretains the special electrical properties when inserted into a magneticfield,

and at the same time have an overall electrical conductivityconsiderably higher than that of bismuth. The special resistivitycharacteristics of bismuth can be readily destroyed by slight impuritiesor other foreign matter which may enter into the space lattice of thebismuth crystals. The fact that bismuth has a rhombohedral hexagonalcrystal lattice further complicates matters because the resistivitybecomes partly a function of crystal orientation.

According to one preferred embodiment of the invention the metal ormetals whose properties are to be improved is comminuted to powder ofconventional grain size. The powdered metal is then pressed into thedesired shape and heated at a temperature and for a time sufficient toproduce sintering of the powder. copper this temperature is in thevicinity of about 975 C. and the time is about 30 minutes. Preferablythe heating is effected in a reducing atmosphere, such as hydrogen, forexample. After satisfactory sintering of the powder, metallic bismuth isbrought into contact with the mass and the heating is continued at asomewhat lower temperature, e. g., about 625 C., also preferably in ahydrogen atmosphere, until the sintered mass has been sufficientlypermeated or impregnated by the bismuth. While the bismuth may be in thesolid form, we prefer the powdered form.

In the impregnation process it is possible to use fairly hightemperatures and thereby improve both fluidity and wettability becauseof the wide spread that exists between the melting point of bismuth(271.3 C.) and the boiling point (1450 0.).

It is possible to produce rather dense composite structures when usingbismuth because the latter expands during solidification while othermetals usually contract during such a phase change. The amount ofexpansion can be calculated from the density in the solid state (9.781g./cc.') and the density in the liquid state which is 10.04 g./ cc. atthe melting point. The volume change in melting therefore amounts toabout 3.4% expressed in percent of the liquid metal.

In an alternative embodiment of the invention, both the metal or metalswhose properties are to be improved and the bismuth may be mixed, in

powder form, e. g., in a. ball mill, for about three hours, the mixturepressed into shape and sintered as described above, for about one hour.A suitable proportion of the mixture is about 40% to bismuth, thebalance being the metal to be improved, but obviously this ratio may bevaried, as desired, and the sintering of the bar or In the case ofarticle may be effected in contact with additional bismuth material,either powdered or solid. Most of the useful compositions containbetween 5 and 60% bismuth. The fact that the compacts made by thepresent procedure will result in a structure of two distinct andseparate phases, on being the bismuth and the other being a soft matrixsuch as copper or silver provides a structure which has excellentpossibilities for bearing applications. Although bismuth has a brittlestructure at room temperature, at slightly elevated temperature it isquite plastic and ductile. At a temperature of about 100 0., forinstance, the metal can be readily extruded into fine wires or'othershapes. In bearings we have operating temperatures which raise theductility and plasticity of bismuth to a point where it becomes wellsuited for antifriction purposes. Furthermore, the complete insolubilityof iron and bismuth will prevent any cold welding of the bearing to theshaft.

It the new compacts are to be used for rectifying discs it is possibleto subject them to chemical treatment in order to form oxides, sulfides,selenides or tellurides, bismuth forming stable comp unds of all of theabove mentioned combinations.

While a preferred embodiment of the invention has been described herein,it is understood that considerable variation may be made in the method01' procedure and th combination of elements without departing from thespirit or the invention as indicated in the appended claim.

I claim:

An operative element 01' an electrical control system which element ischaracterized by a relatively high but variable overall electricalconduc tivity, this conductivity varying in use in response to varyingapplied magnetic fields, said element comprising a sintered component 01the group consisting of copper and silver, and a bismuth componentdispersed in said sintered component, said bismuth component comprising5 to 60% of said element.

FRANZ R. HENSEL.

